On a conventional optical disk typified by the CD and the DVD, information is recorded by forming optically detectable pits or marks on the medium, and the information is reproduced by detecting the pits or marks by optical means.
The pits or marks recorded on the conventional optical disk are read as an electric signal by an optical pickup, the waveform of the pits or marks is judged to be binary information of 1 or 0 by a judging circuit, and then the result is outputted.
Specifically, a repeated waveform referred to as an “eye pattern” is detected from the optical pickup, and then the waveform is compared with a predetermined threshold value, thereby decoded as binary information and restored to original digital information.
In such a conventional reproducing method, an upper limit of fineness (spatial frequency) of a detectable signal is determined by wavelength of a laser beam and a numerical aperture (NA) of an objective lens used in reproduction.
Specifically, as a recorded signal becomes finer, amplitude of the reproduced signal is decreased, and when an upper limit of spatial frequency is reached, the recorded signal cannot be detected at all. Such change in spatial frequency characteristics is generally referred to as MTF (Modulation Transfer Function).
As an example, FIG. 11 shows MTF characteristics of an optical pickup used for DVD. The axis of ordinates in FIG. 11 indicates amplitude (gain) of a reproduced signal, and the axis of abscissas indicates spatial frequency.
As is clear from FIG. 11, in the case of DVD, detection is possible up to a spatial frequency of about 1700 lines per millimeter.
In the conventional optical disk recording method, an upper limit value of MTF as described above limits fineness recordable on an optical disk. As a result, recording density of an optical disk is fixed.
Hence, optical disk high-density recording proposed thus far raises such a limit of MTF in many cases. Specifically, optical disk high-density recording proposed thus far further shortens the wavelength of a laser or increases the NA.
As a result of these, it is possible to further raise the frequency limit of MTF, and thus increase recording density.
However, the wavelength of a laser has a limit of about 400 nm, and when the wavelength is made shorter than the limit, the light does not pass through a plastic substrate.
In addition, increasing the NA requires reducing a space between a surface of the optical disk and the objective lens. Data reading therefore becomes unreliable when there is dust on the surface of the disk or the like.
Multilevel recording has also been proposed which records information as multilevel information rather than as a binary value of 1 or 0.
In Japanese Patent Laid-Open No. Sho 61-94244, for example, an increase in recording capacity is made possible by irradiating an identical spot on a recording medium having a recording layer in which a hole is formed by light irradiation with a varied number of beams according to information contents, thus forming holes of different depths and thereby recording multilevel information.
That is, the invention realizes multilevel recording by changing the number of beam spots applied to the same spot and thereby changing the depth of the hole stepwise.
An information recording apparatus disclosed in Japanese Patent Laid-Open No. Hei 2-31329 modulates power of a light beam to a plurality of levels, phase-changes a phase change recording medium to a plurality of stages, and thus records information as a multistage, or multilevel signal.
In Japanese Patent Laid-Open No. Hei 4-38088, multilevel information is recorded by changing a coordination environment in a metallic-complex. When an octahedral coordination is used as a typical example, the invention enables multilevel recording of up to six levels using up to six changes.
However, when multilevel recording (multistage recording) as described above is applied, a signal-to-noise ratio (SNR) of a medium needs to be improved.
In addition, a direct-current component of a recording signal cannot be eliminated in multilevel recording. Therefore, it is known that when overall level of a reproduced signal is raised or lowered, the signal is decoded into information different from the original information and thus error rate is drastically deteriorated.
The MTF shown in FIG. 11 has the axis of abscissas as frequency. Frequency used in handling an electric signal assumes a plus value at all times.
However, as for optical resolution, there are a plus spatial frequency and a minus spatial frequency. Specifically, as shown in FIG. 12, not only a plus spatial frequency region but also a minus spatial frequency region is actually present. Describing the plus and minus spatial frequency regions as light diffraction, the plus and minus spatial frequency regions correspond to light diffracted forward and light diffracted backward with respect to rotation of a disk.
It is accordingly an object of the present invention to provide an optical disk medium and an apparatus and a method for optical disk recording/reproduction that separate the MTF shown in FIG. 11 into a plus frequency region and a minus frequency region and use the plus frequency region and the minus frequency region to enable an effectively usable frequency region to be doubled in effect and thus enable recording density to be increased.
That is, the present invention can increase the density without the need for an increased NA of an objective lens and a shorter wavelength and without substantially changing an optical pickup and optical disk manufacturing technology currently in use. Also, the present invention enables high-density recording at a lower SNR as compared with multilevel recording.